Filters in filter wheels have long been known to produce longitudinal chromatic aberrations. That is, the angle of non-collimated light passing through a filter of a specific narrow bandpass, will be altered depending upon the actual wavelengths of the light passing through the filter. This is pursuant to the law of refraction. Because a filter's purpose is to produce differing bandpass wavelengths, one for each filter, in an analyzer reflectometer, for example, the result when using non-collimated light through filters of equal thicknesses is to alter at each filter the focal length from the filter wheel to the next optical element. If that next element is a detector, the image to be detected will be in or out of focus at the detector, depending on which filter is in place, so long as the detector's position is fixed (as it usually is).
This totally unsatisfactory state of affairs has been corrected, in the past, by ensuring that only collimated light passes through the filter wheel. That is, there is no longitudinal chromatic aberration, by definition, when using collimated light. However, this in turn requires care in collimating the light, such as with an excessively large number of collimating lenses on both sides of the filter wheel. For example, two collimating lenses are usually used upstream, and four converging lenses are usually used downstream.
Although such an arrangement has worked very well in the past, there is one drawback--the filter wheel must be between a set of lenses. Hence, it has not been readily feasible to reduce the path length of light as it proceeds from a "source" (usually, the object being illuminated) to the receiving element (usually a detector). That is, the required lenses cannot be crammed into a shorter path length. In contrast, it would be desirable to have a much shorter light path, so as to reduce the size of the reflectometer.
Hence, it has been a long-standing problem, where filter wheels have to be used, that the path length has been excessively long due to the lenses needed both upstream and downstream of the filter wheel.